I've got a 1.5" layer of XPS foam against a cold concrete wall with
2x4 framing inside that. I temporarily placed a couple pieces of R12
fiberglass batt in the 2x4 wall. When I removed the batts the next
day, there was a considerable amount of 'sweat' on the XPS. No
moisture was detected on the XPS where it was left uncovered.
The area is not directly heated and currently sits around 55 degrees.
Exterior temperature is closer to 45 degrees.
Any ideas why condensation would occur between the two materials?
It's not obvious to me. The plan was to fully insulate the walls with
fiberglass and then cover with a plastic vapour barrier...now I'm not
so sure that's a good idea.
That sounds reasonable offhand, bee. Also there might be the possibility
that any air flow has been stopped or nearly so, removing any way for
moisture to seep out so it condenses first.
I wonder if, in that area, if only unfaced insulation or even none, wouldn't
be better to do. Concrete, contrary to what a lot of people think, and its
appearance, will in fact leak moisture in through it, so stopping that
possibility alone can't really be done at any reasonable cost.
Yes, the XPS is essentially a vapour barrier. What I don't understand
is why presence of the batt would create a temperature differential
large enough to cause condensation (more specificially, wouldn't
temperature difference between air and exposed XPS be similar)?
My assumption here is that moisture is coming from the ambient air
(which has relatively high humitidy...air exchanger is currently on
the fritz). Once covered with a plastic vapour barrier, that ambient
moisture will no longer be able to condense inside the wall. Valid
thought? FWIW, I've been instructed to cut slits in the XPS to reduce
the double vapour barrier effect.
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